The present invention relates to an optical apparatus having an autofocus (“AF”) control using a photoelectric conversion element (“PCE”).
A single lens reflex type digital still camera or digital single lens reflex (“DSLR”) camera adopts the so-called pupil slicing focus detection for precise focusing upon a quickly moving subject.
FIG. 7 shows a schematic structure of a conventional DSLR camera system. When a photographer observes a subject through an eyepiece 104, part of light 110 from the subject that transmits an image-taking lens 120 is reflected on a main mirror 101 and forms a subject image on a focusing glass 102 in a camera body 100. The subject image formed on the focusing glass 102 is guided to the photographer's eye via a penta prism 103 and the eyepiece 104.
Part of the light 110 from the subject passes through the main mirror 101, and is reflected on a sub-mirror 105 and guided to a focus detection unit 106. The focus detection unit 106 includes a field lens, a mirror, a stop mask, a secondary imaging lens, and a light-receiving sensor. The light-receiving sensor receives the light that passes different pupil areas on the image-taking lens 120, and an image signal is output from each of a pair of or plural pairs of line sensors in the light-receiving sensor. A focusing state (such as a defocus direction and a defocus amount) of the image-taking lens 120 can be detected based on a phase difference of this image signal. In addition, a driving direction and driving amount of a focus lens 123 in the image-taking lens 120 are calculated from the detected focusing state, and focus is obtained by driving the focus lens 123.
In the image-taking time, both the main mirror 101 and sub-mirror 105 retreat from the optical path and the light from the subject which has passed the image-taking lens 120 is guided to the image sensor 108.
The pupil slicing focus detection method of a through the taking lens (“TTL”) phase difference detection (“PDD”) requires a sensor dedicated for a focus detection and a secondary imaging optical system, and thus has difficulties in reducing a size and cost of the camera.
Accordingly, a digital still camera has recently been proposed which utilizes a subject taking image sensor for the TTL PDD and pupil slicing focus detection. For example, Japanese Patent Application, Publication No. (“JP”) 9-43507 inserts, near a pupil of the image-taking lens, a mask that transmits the light from part of the pupil, and detects focus using a signal from the image sensor corresponding to two images by switching an opening position of the mask. Another focus detection system, proposed in JP 2004-46132, uses part of the image sensor as an AF sensor area, and guides to the area two lights split by a split image prism provided in the imaging optical system.
JP 4-147207 discloses another structure for the TTL PDD AF by arranging a holographic optical element closer to the object side.
However, the focus detection method proposed in JP 9-43507 changes the mask opening position, reads twice an image of the light that passes different pupil areas in the image-taking lens as an output (image signal) of the image sensor, compares these image signals. It takes a relatively long time to switch the mask opening position and to read two image signals from the image sensor. This focus detection method is rather unsuitable for a quickly moving subject.
A method that uses a split image prism proposed in JP 2004-46132 needs a larger image than the split image prism, different from the TTL PDD. In addition, unless an image on the boundary of the split image prism has a linear shape, this method determines that it is defocused even if it is focused. Therefore, this method has a limited focus detection capability or is inferior to the TTL PDD.
A method that uses the holographic optical element proposed in JP 4-147207 is similar to the TTL PDD in principle, but the holographic optical element possesses large color dispersion that causes an image to contain aberration in forming two AF images in the pupil slicing direction, which are important to a determination of the focusing state. Thus, this method is not practicable for the focus detection.